Chemical symbols D and T

[merkys]

OPTIMADE specification v1.0.0 contains quite vague guidelines about allowed values for chemical symbols. Implicitly it is understood that symbols from the periodic table + X + vacancy are allowed. However, deuterium (D) and tritium (T), which are present in the COD, are two common symbols not appearing in the periodic table. Maybe the validator could allow them too, or should they better be represented as hydrogen atoms with mass of 2 (D) or 3 (T)?

[ml-evs]

I think this is something you'll have to bring up in the specification discussion; it seems clear in the spec that species->chemical_symbols can only contain element symbols, X and vacancy. We also apply the hard filter that elements only contains elements.

I think you are completely free to include D and T as labels in species_at_sites, but then they would have to be defined as {"name": "D", "chemical_symbols": "H", "mass": 2} in the species field as you suggested.

The PR on chemical formula validation in #547 will also exclude non-elemental symbols in e.g. chemical_formula_reduced, would you want to include D and T there too?

[CasperWA]

This is an extremely important point, and something I have already touched upon in some technical PRs by @ml-evs that introduced these hard constraints.

But as @ml-evs also states above, this is an issue of the specification, not this implementation, and something that should be raised in the specification repository and possibly at the next meeting.

[merkys]

I guess the specification is indeed the right place to discuss these elements. I will open an opened Materials-Consortia/OPTIMADE#327 issue there.

The PR on chemical formula validation in #547 will also exclude non-elemental symbols in e.g. chemical_formula_reduced, would you want to include D and T there too?

Good catch. Indeed, I would want to see D and T in formulae as well.

[ml-evs]

I think we can close this following the discussion in the meeting today; any further discussion can be had at the specification issue linked above.